Substitute for Nuts in Cake Recipe
I like the dried fruit suggestions, so perhaps choose a dried fruit that your father really likes. I think dried berries would lend themselves well to a sour cream cake and would look pretty and summery.
Of course, you could go tropical and use bits of dried mango, pineapple, or papaya w/ some shredded coconut (if that's allowed). I would personally be into something like bits of candied ginger and dried apricot.
Not sure if your recipe calls for dusting the nuts in a little flour so they don't sink to the bottom while baking, but I've seen recipes call for doing this w/ dried fruit. Good luck. I'm making my FIL a cherry and nectarine pie, unless I change my mind again.
chopped dried cherries, cranberries or blueberries?
maybe just some orange zest or candied orange peel? maybe a citrus or jam glaze?
some cinnamon crumbly topping (a la coffee cake)
how about make it plain and add macerated strawberries or raspberries with creme fraiche or whipped cream?
maybe a little gran marnier?
nmmmmmm... let us know what you decided and how it turned out.